What are Blender nightly builds?
Blender is under continuous development, with new features and bug fixes being added to the code as soon as they meet the criteria for ready. The codebase is the built nightly and the builds are available at https://builder.blender.org/download/. 

This allows users to preview and test what will be in the next release, but should be used with care, as these builds are not fully tested and so may contain significant bugs.